Electrodeionization (EDI) systems are used to remove ions from liquids, especially water. These systems require a power supply that applies voltage to the EDI module to refine industrial process water to ultra-high purity for use in the power, microelectronics, food, chemical, pharmaceutical, and other industries.
In typical electrodeionization devices, electrical current flows through the bed of ion exchange resin. The resin bed is contained on either side, perpendicular to the flowing current, by ion-exchange membranes. The current passes through the bed via ion migration through both the solution and the ion-exchange beads, with water dissociation occurring at the anion-cation, bead-bead and bead-membrane interfaces. The electrical potential required to pass this current is dependent on the mobility of the ions in the ion-exchange phase of the beads and the membrane, the mobility of the ions in the solution surrounding the beads and the potential required for water dissociation.
In an electrodeionization device, the impurity ions are fed into one end of the bed, perpendicular to the applied current flow and the pure water exits the other end of the ion-exchange bed. This situation sets up a gradient for the impurity ions from the inlet to the outlet of the bed, e.g., with a NaHCO3− feed the ion-exchange media at the inlet will be predominantly in the Na+ and HCO3− forms, and will gradually decrease in Na+ and HCO3− concentrations towards the outlet. In the outlet region the ion-exchange media are predominantly in the regenerated H+ and OH− forms. In a mixed or layered diluting chamber electrodeionization device processing a normal reverse osmosis permeate, this gradient in speciation, from inlet to outlet, results in the inlet of the device being less conductive than the outlet due to the relative mobilities of Na+ and HCO3− being much less than those of H+ and OH−. Consequently, when a constant potential is applied across the EDI device the current flowing at the outlet is significantly larger than the current at the inlet.
There are several factors which are known to influence the mobility of the ions in a bed of ion-exchange media, such as: (1) the nature of the ionic species, i.e., for cations, H+ vs. Na+ vs. Ca2+; (2) the nature of the ion-exchange material including the percentage cross-linkage, concentration of ion-exchange sites, distribution of ion-exchange sites, and the bead surface structure; (3) the concentration of the ionic species; (4) the quantity of anion-cation bead-bead interfaces; (5) the quality of the anion-cation bead-bead interfaces; (6) the composition of the solvent being processed through the device; and (7) temperature.
It is known that the ability of the EDI device to remove impurity ions and thus produce high purity product water is significantly dependent on the distribution of the regeneration current. Attempts have been made to modify the conductivity of the anion and cation ion exchange phases in an EDI device to improve deionization performance such as those described in U.S. Pat. Nos. 6,284,124 and 6,514,398 to DiMascio et al. The DiMascio et al. devices are characterized by an ion-depleting compartment having alternating layers of ion exchange resin material wherein a dopant material is added to one of the layers to reduce the difference in conductivity between the alternating layers.
What is not taught or suggested by the prior art is an improved EDI device comprising at least one resistive component coupled to the bead-membrane interface near the outlet region of the device to increase the electrical resistance of the outlet region with respect to the inlet region of the device in a relatively simple and cost effective manner, thus increasing the current distribution at the inlet region of the device with respect to the outlet region of the device and enhancing the overall deionization performance of the device. It would also be desirable to have an improved EDI device which is easily adaptable to a variety of different applications.